CPI - Concatenate two properties

I am trying to concatenate two properties with in the expression of the content modifier. However in the Bill of lading I am seeing the value below. Can anyone help with correct syntax to get the desired value in the field. Appreciate your time.

BillOfLading> <string>123344</string>+/+<string>0</string> </BillOfLading>

Instead of

BillOfLading>

123344/0

</BillOfLading>

<value><br>       <BillOfLading><br>            <string>${property.BillOfLading}</string>+/+<string>${header.CamelSplitIndex}</string><br>       </BillOfLading><br>           ${in.body}<br></value>